I have a Stalker at home,but since I use to throw Teebirds I was curious about getting a Rival.

Don't really want to go the Teebird route so the Rival sounds like a good choice.

I like my Stalker,but I don't think it is Stable enough to go between my Trident and River.

Is the Rival a good choice and if so which plastic is the best? I'm a little new to Legacy so I'm not sure about the plastics.

Rivals are more like Eagle-X's. Some of the heavier Pinnacle Rivals will be close to a TeeBird though. I loved the flight of the Rivals, but hated the plastic which is a bit too stiff for my liking. However, the new Pinnacle Patriots are supposedly less stiff. Stalkers are shorter than the Rival by a little bit. Stalkers in my opinion were longer Buzzes.

The Icon Rivals were in my opinion beefy Eagles. The plastic was less stiff and better feeling then the Pinnacle I had. I still felt it was too stiff for my liking throwing them in the cold Michigan winters.

I even tried a Legend line which is like Legacy's Gold Line, and that was much better feeling and a more TL like flight which I really liked , but the green color was not so good. I would keep an eye out because Legacy is releasing the Legend line in brighter colors soon. They will be great sellers.